Microwave oven apparatus



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INVENT .KARL FRITZ" BY ATTORNEYS June 15, 1965 Tz 3,189,722

- MICROWAVE OVEN APPARATUS Filed Sept. 21, 1962 2 Sheets-Sheet 2 v 0 0//42 Z o I 0 4 1p /5 24 /Zb' F I G. 8 /2 Z5 INVENT KARL FRI BY Wa-L,M4#M

ATTORNEYS United States Patent 3,189,722 MICROWAVE OVEN APPARATUS KarlFritz, Freibnrg im Breisgau, Germany, assignor to MIWAG MikrowellenAktien Gesellschaft, a corporation of the Swiss Confederation FiledSept. 21, 1962, Ser. No. 225,225 2 Claims. (Cl. 219-1055) The presentinvention relates in general to heating by electrical energy and moreparticularly to ovens utilizing microwave energy for heating purposes.

In order to obtain an even distribution of heat through an object ormaterial heated by microwave energy, it is common practice to provideapparatus for ensuring a regular distribution of microwave energy in ametallic enclosure and to compensate for residual inequalities therein.In the past, devices such as a rotating radiator or reflector, sometimestermed a mode mixer, have been used to cause the standing wave patternset up by the microwave field to move over the object being heated whiledevices such as a rocking or rotating support or a conveyor belt orchain have been used to move the heated object in the microwave field.

In practice, it has been found, where the microwave energy in theheating enclosure assumes a stationary and regular pattern, that causingrelative movement of the field and the object is not enough to give asufficiently even rate of heat-ing throughout the volume of somesubstances. This is especially true With electrically nonisotropicsubstances. For example, frozen organic materials (e.g., frozen foods)usually consist of an agglomeration of irregularly oriented ice crystalsand particles of other matter. The electrical propert es (absorptionfactor, loss angle, dielectric constant) and other physicalcharacteristics, such as thermal conductivity, of certain electricallynon-isotropic substances are different from particle to particle andvariable to a marked degree with temperature during treatment when thesubstances are changes from the frozen to the thawed state, or duringthe process of desiccation. Moreover, some substances have propertieswhich vary within the particles themselves.

Even in a microwave field of homogeneous amplitude distribution whererelative movement of the field and object is utilized in order toequalize heating, there can still be unequal distribution of the rate ofthawing when certain materials are treated, because the rate of heatingof diiferent parts of the material will depend to a considerable degreeon the plane of polarization of the microwave field to which they aresubjected. Thus, when heating is to be carried out in a short time byuse of a high microwave power level, hot spots may develop in thematerial at a time when other parts of same are still cold. For certainmaterials, this result is of little practical importance. For example,deep-frozen raw foods are customarily thawed and cooked in onecontinuous operation, and as they are kept at the boiling point muchlonger than the time spent in thawing, heat conduction within thematerial equalizes final temperature soon enough for all parts to beadequately done.

In other situations, such as in industrial and scientific applications,hot spots resulting from inequalities in the rate of thawing cannot betolerated. For example, in the case of many chemical products andbiological Patented June 15, 1965 materials, no part of the materialmust, at any time during treatment, even temporarily be heated above thetemperature, which is often quite low, at which chemical decomposition,inactivation of ferments or antibodies, denaturizing of proteins, orother deleterious act on, can occur.

The principal object of the present invention is to obtain an equal rateof heating throughout an object heated in a microwave oven. An advantageof the present invention is that uniform heating is achieved inmaterials which are non-homogeneous or anistr-opic or both, wherein theheating effect produced in the material depends not only on thedistribution of the microwave field but also on the plane ofpolarization of the incident microwave energy. Further, relativemovement of the microwave field pattern and the object being heatedserves not only to average the field amplitude in which the object isexposed over the time of heating, but such relative movement also servesat the same time to expose the object to waves of a differentpolarization so that different responses of different parts of theobject to waves polarized in different planes are averaged out as well.By employing the invention, non-homogeneous objects are heated as evenlyas homogeneous objects, even where the heating period is so short thatheat conduction within the material cannot play an important role inequalizing temperature. A power level which might cause hot spots innon-isotropic materials with conventional apparatus may be used withoutdetrimental effects in apparatus constructed according to the invention;therefore, when large quantities of foods are to be heated in a shortinterval, as in .a restaurant during peak business hours, valuable timecan be saved. Finally, in the case of anistropic or non-homogeneousmaterials which are sensitive to rapid heating to the extent that thepermissible power level with conventional apparatus is so low as torender microwave heating parctically useless, by employing equipment andprocedures according to the invention, enough power can be used to heatsuch mateterials rapidly without special regard to the r physical andelectrical configuration.

The invention resides in an apparatus having a metallically confinedheating chamber into which microwave energy is fed. A pair of polarizinggrids are arranged perpendicular to the direction of wave propagation ata distance from the front and rear walls of the chamber corresponding toa quarter wavelength of the energy of the enclosure. Further, the planesof polarization of the grids are at right angles to each other. Theincoming wave energy is split into two orthogonally polarizedcomponents, one component setting up a pattern of standing waves betweenone wall and the opposing grid from which it is reflected and the othercomponent an identical pattern between the other wall and the othergrid. As both patterns are at right angles to each other, there is nomutual interference between the two polarized wave patterns, and as theyare displaced axially by a quarter wavelength, a maximum of one patterncoincides with the minimum of the other. The wave pattern of the mutualinterpenetration area of the heating chamber is not only subject toperiodic variations of field strength, but also to periodic changes inpolarization, so that an object in the chamber is heated evenly. In oneembodiment of the invention, microwave energy is fed into the heatingenclosure through a horizintal coupling slot, at an angle of 45 to thepolarizing grids. In another embodiment of the invention, the energy isfed into the heating enclosure through a pair of slots whose planes ofpolarization are perpendicular to each other, each slot being parallelto one of the polarizing grids. In a further embodiment of theinvention, energy is fed into the heating enclosure by means of ahalf-dipole antenna. The two polarizing grids are rotated synchronously.As the grids rotate, they alternately reflect or pass polarizedradiation from the antenna so that a field pattern is alternately set upbetween one of the walls and one of the grids. In still anotherembodiment of the invention, the plane of polarization of the field ismoved by means of a rotating antenna, alternately setting up fieldpatterns between one of the walls and one of the grids.

Referring now to the drawings,

FIG. 1 is a schematic representation of a heating chamber havingpolarizing grids adjacent its end walls;

FIGS. 2 and 3 show the standing wave patterns set up in the heatingchamber by the two orthogonally polarized components of the microwaveenergy;

FIG. 4 shows the resultant distribution of microwave energy in theheating chamber;

FIG. 5 schematically depicts an embodiment of the in vention in whichwave energy is fed into the working space within the chamber throughorthogonally disposed coupling slots;

FIG. 6 depicts a type of polarizing grid which may be used in theinvention;

FIG. 7 illustrates another embodiment of the invention in whichpolarizing grids in a working space are continuously rotated; and

FIG. 8 depicts still another embodiment of the invention wherein anantenna feeding wave energy into a working space is continuouslyrotated.

Referring now to FIG. 1, which depicts en ambodiment of the invention inschematic form, there is shown a metallically closed heating chamber 2of an oven into which microwave energy from a Waveguide 4 is fed througha radiating element such as a horizontal extending slot 6 in the endwall of the chamber. The polarization of the wave emanating from theslot 6 is perpendicular to the plane in which the length of the slot 6extends. A pair of polarizing grids 8 and 12 are arrangedperpendicularly to the direction of wave propagation in the heatingchamber at a distance from the front and rear walls thereof,corresponding to a quarter wavelength of the energy in the enclosure.The planes of polarization of the grids 8 and 12 are at right angles toeach other and at an angle of 45 to the slot 6. The incoming wave energyis split into two components whose planes of polarization are at rightangles to each other, one component setting up the standing wave patterndepicted in FIG. 2 between one wall and the grid from which it isreflected and the other component setting up a similar pattern depictedin FIG. 3 between the other wall and the other grid. As each pattern isconstituted by wave energy which is polarized at right angles withrespect to the Wave energy of the other pattern, there is no mutualinterference between the two polarized Wave patterns, and as they aredisplaced axially by a quarter Wavelength, a maximum of one patterncoincides with the minimum of the other pattern.

FIG. 4 shows the microwave energy distribution, which is the resultantof the wave patterns of FIGS. 2 and 3, in the mutual interpenetrationarea which is the actual working space of the enclosure 2. An objectmoved in this area is not only subject to periodic variations of fieldstrength, but also to periodic changes in polarization, so that theobject is heated evenly in all of its parts, regardless of any directionof electrical predilection; viz., electrical anisotropy.

Referring now to FIG. 5, there is shown an alternative embodiment of thedevice of FIG. 1. In the apparatus of FIG. 5, the microwave energyavailable from waveguides 22 and 24 is fed into the heating enclosurethrough a pair of radiating elements, such as slots 26 and 28, Whoseplanes of energy polarization are at right angles to each other. Slot 26coincides with and is, preferably, an aperture in end wall 30; itsradiation passes freely through grid 32, the bars of which are set atright angles to the plane of polarization of the energy emanating fromslot 26, that is, parallel to the length of the slot 26. Slot 28, whichis at right angles to slot 26, lies in the plane of the grid 32. Thelatter grid, as well as grid 34, are mounted one-quarter wavelength fromthe end walls of the chamber. Grid 34, the bars of which are set atright angles to those of grid 32, allows the radiated polarized energyfrom slot 28 to pass, but reflects the radiated energy from slot 26. Thedevice of FIG. 5 operates so that the two separate interpenetratingfield patterns are set up as in FIGS. 2 and 3, each offset by a quarterwavelength so that the maxima of one standing wave falls on the minimumof the other standing wave, while their planes of polarization are atright angles to each other. The arrangement of FIG. 5 allows a somewhatmore even distribution of field energy than the device of FIG. 1.

In the embodiments of FIGS. 1 and 5, one of the ends of the enclosures,preferably, is the door of the oven, and the door is made approximatelya quarter wavelength deep so that it carries the adjacent grid. In thedevice of FIG. 1, the grid 12 and the portion of the chamber to itsright is hinged to swing downwardly as a unit to form the ovens door,whereas in the device of FIG. 5 the grid 34 and the portion of thechamber to its right is similarly hinged to provide a door for thatembodiment. In this manner, when the door is opened, the grid swingswith the door so that access to the interior of the oven is unhindered.

Referring now to FIG. 6, there is shown a ditfcrent type of gridconstruction which can be used to form the grids 12 and 34 in theembodiments of FIGS. 1 and 5. When higher order modes occur in theheating chamber of the devices of FIGS. 1 and 5, the subdivision of thegrids is made according to the mode which carries a greater amount ofenergy. The simple grids shown in FIG. 5 are used with the fundamentalor lower order mode. FIG. 6 indicates a grid 42 intended to be used witha higher order mode.

As previously discussed, the invention contemplates including provisionsfor relative movement of the object heated in the oven and the fieldpatterns therein. While mechanical movement of the object may be used toobtain even heating since the loci of equal field intensity andcorresponding polarization would be parallel to the three dimensions ofthe enclosure, best results are obtained when the object is moved in adirection which is not parallel to any of the three spatial coordinatesof the enclosure, in a way sometimes referred to as wind-blown. Theactual mechanical arrangement most suitable in a given case is describedin copending application No. 225,222. On the other hand, methods ofmoving the field pattern about a stationary object is thought to be themost suitable way and are contemplated by the invention. One obvious wayis to move the polarization grids; another, to move the radiatingelements; a third, to move thed end wall surfaces which reflect energyback to the gri s.

FIGS. 7, 7a, and 7b depict an embodiment of the invention wherein thepolarizing grids are continuously rotated. In this embodiment, themetallic enclosure 52 is a cylinder. Microwave energy is fed into theenclosure 52 from a generator 54 by means of a half dipole antenna 56 Ata distance of a quarter wavelentgh from the end faces 58 and 62 of thecylindrical enclosure are mounted polarizing grids 64 and 66, the barsof which are perpendicular to each other. The two grids 64, 66 aremounted on a pair of gears 68, 72, respectively. The gears 68, 72

mesh with a second pair of gears 74, 76, respectively. In order torotate the grids 64, 66 synchronously, a motor 78 drives a pair ofshafts 82, 84. The shaft 82 is connected through a bearing 86 to thegear 74.

The end portion 88 of enclosure 52 containing the grid 66 is slidablypositioned as can be seen by the dotted lines in FIG. 7, thus allowingaccess to the enclosure 52. The shaft 84 passes through a bearing 92 andis connected to a fork 93. The gear 70 is connected to the fork 93 bymeans of a rod 94 which contains a pin 96 at one end thereof. The pin 96is movable in a slot 98 cut in the fork. The end portion 88 is pivotallymounted on brackets 100 so that the end portion may be supported whenopened. As the grids rotate, they will alternately reflects or pass thepolarized radiation from the antenna so that a field pattern isalternately set up between wall 58 and grid 66, or wall 62 and grid 64.Not only is the field thus moved continually from left to right and viceversa, but its plane of polarization will vary through 90 at the sametime so that the object being heated in the oven is exposed to microwavefields of varying intensity and polarization, averaging over the time oftreatment. Thus, the device of FIG. 7 provides even heating through allparts of the object and in all directions, even in the presence ofpronounced standing wave formation. It will be noted that, while thefield pattern is moved, the electrical length of the enclosure remainsunchanged, an obvious advantage when designing for a particularconfiguration of field patterns.

Referring now to FIG. 8, there is shown an embodiment for obtaining amoving field with variation of the plane of polarization. In thisembodiment, a pair of polarization grids 112 and 114 are stationary, andthe plane of polarization of the field is moved by means of a rotatingantenna 116. The polarizing grids 112 and 114 are mounted a quarterwavelength away from opposite end walls 118 and 122 of the enclosure 124and have their polarizing elements at right angles to each other. Theenclosure is cylindrical in shape, one of its end faces constituting adoor such as end wall 122 to which the grid 114 is fixed. The member 122is pivotally mounted on a bracket 126 and contains a flange 128 whichmates with flange 132. The antenna consists of an L-shaped rod 134 whichforms one dipole element, and at the same time, the coupling member to awaveguide 136 onto which microwave energy is fed from a generator 138,and a second L-shaped rod 142, one end of which is mechanically andelectrically connected to a metal disk 144. The two rods 134 and 142 aresecured together by an insulating member 146. The disk 144 is insulatedfrom the wall of waveguide 136, thereby forming a grounding capacitor.Rod 134 carries an insulating extension 148, which is rotatedmechanically, for example, by means of a driving pulley 152. Acharacteristic feature of the arrangement of FIG. 8 is that the antennarotates inside a cylindrical waveguide cavity 153 which is coaxial withthe cylindrical wall working enclosure 124.

The operation of the device of FIG. 8 is as follows: As the antenna isrotated, field patterns are alternately set up between polarizing grid112 and end wall 122, or polarizing grid 114 and end wall 118. Theworking space between the two grids is thus the interpenetration area oftwo similar field patterns, offset by a quarter wavelength, moving backand forth with continuous variation of the direction of polarization.The object being heated is thus exposed, over the time of treatment, tomicrowave energy of varying amplitude and polarization. Thus,inequalities in heating due to anisotropy in the material are averagedout. As can be readily seen, the electrical length of the enclosure 124is held constant.

The arrangements described provide polarization only in a planesymmetrical about the longitudinal axis of the enclosure so thatadditional provision would seem necessary in order to obtain fieldpattern polarization in all directions. It is found, however, thathigher modes of 6 oscillation occur in the enclosure of the usualdimensions. These modes have longitudinal field components, the totalintensity of which is suflicient to insure that the object is exposed tosubstantially equal amounts of microwave energy polarized in everydirection.

It is to be understood that although thawing of frozen foods is theapplication most frequently referred to in the present application, thisis merely because it is the problem most frequently encountered. Theinvention is, obviously, useful for any product in which the material tobe heated is, or becomes, anisotropic during any stage of treatment,such as freeze-drying, vacuum drying, crystallization from liquids,melting of crystals, treatment of Wood and fibers, drying of emulsionsor suspensions, curing of chemical mixtures, and numerous others.

It is also obvious that apparatus according to the invention is by nomeans restricted in its application to treatment of anisotropicmaterials, but can also be used equally well for heating any kind ofmaterial, homogeneous or not, to any desired temperature. In otherwords, While conventional apparatus may constitute a good heating oven,but not be quite satisfactory when used for thawing, apparatusincorporating the characteristic features of the invention willconstitute a most satisfactory heating oven, and in addition, be able tohandle frozen foods much better than ordinary equipment. The inventionis thus of universal application in extending the range of usefulness ofany kind of microwave heating apparatus.

Obviously, many other modifications and variations of the presentinvention are possible in the light of the foregoing teachings. It is tobe understood, therefore, that the invention is not limited in itsapplication to the details of construction and arrangement of partsspecifically described or illustrated, and that within the scope of theappended claims, it may be practiced otherwise than as specificallydescribed or illustrated.

I claim:

1. In a microwave oven of the type having:

an enclosure for confining microwave energy, the enclosure having a pairof oppositely disposed wave energy reflecting walls;

a source of microwave energy;

apparatus for coupling energy from the source to the enclosure;

and a pair of polarizing grids in the enclosure, each grid reflectingwave energy polarized in a first plane and being transparent to waveenergy polarized in a second plane perpendicular to the first plane; thegrids being disposed at right angles to one another, the grids beingspaced from the reflecting walls to cause the establishment in theenclosure of at least two, non-coincident, similar, standing wavepatterns; the improvement comprising:

an antenna forming a part of the apparatus for coupling energy from thesource to the enclosure, the antenna being of the type radiatinglinearly polarized waves;

means for rotating the antenna during operation of the oven;

and means for causing radiation from the antenna to be directed towardthe polarizing grids.

2. In a microwave oven of the type having:

an enclosure for confining microwave energy, the enclosure having a pairof oppositely disposed wave energy reflecting walls;

a source of microwave energy;

apparatus for coupling energy from the source to the enclosure;

and a pair of polarizing grids in the enclosure, each grid reflectingwave energy polarized in a first plane and being transparent to waveenergy polarized in a second plane perpendicular to the first plane, thegrids being spaced from the reflecting walls to cause the establishmentin the enclosure of at least two non-coincident similar standing wavepatterns;

the improvement comprising:

an antenna for radiating polarized waves, the antenna forming a part ofthe apparatus for coupling energy from the source to the enclosure, theantenna being situated between the two polarizing grids wherebypolarized wave energy is simultaneously directed toward the two grids;

and means for synchronously rotating the polarizing grids.

References Cited by the Examiner UNITED STATES PATENTS Wolff 343786Blass et a1 219-10.55 Weil 343-756 Guanella 21910.55 Donnellan et al343756 RICHARD M. WOOD, Primary Examiner.

1. IN A MICROWAVE OVEN OF THE TYPE HAVING: AN ENCLOSURE FOR CONFININGMICROWAVE ENERGY, THE ENCLOSURE HAVING A PAIR OF OPPOSITELY DISPOSEDWAVE ENERGY REFLECTING WALLS; A SOURCE OF MICROWAVE ENERGY; APPARATUSFOR COUPLING ENERGY FROM THE SOURCE TO THE ENCLOSURE; AND A PAIR OFPOLARIZING GRIDS IN THE ENCLOSURE, EACH GRID REFLECTING WAVE ENERGYPOLARIZED IN A FIRST PLANE AND BEING TRANSPARENT TO WAVE ENERGYPOLARIZED IN A SECOND PLANE PERPENDICULAR TO THE FIRST PLANE; THE GRIDSBEING DISPOSED AT RIGHT ANGLES TO ONE ANOTHER, THE GRIDS BEING SPACEDFROM THE REFLECTING WALLS TO CAUSE THE ESTABLISHMENT IN THE ENCLOSURE OFAT LEAST TWO, NON-COINCIDENT, SIMILAR, STANDING WAVE PATTERNS; THEIMPROVEMENT COMPRISING: AN ANTENNA FORMING A PART OF THE APPARATUS FORCOUPLING ENERGY FROM THE SOURCE TO THE ENCLOSURE, THE ANTENNA BEING OFTHE TYPE RADIATING LINEARLY POLARIZED WAVES; MEANS FOR ROTATING THEANTENNA DURING OPERATION OF THE OVEN; AND MEANS FOR CAUSING RADIATIONFROM THE ANTENNA TO BE DIRECTED TOWARD THE POLARIZING GRIDS.